In a fuel storage tank of a motor vehicle that contains fuel, volatile hydrocarbons are continuously escaping. This effect increases with temperature and the agitation or sloshing of the fuel. In motor vehicles driven by internal combustion engines, for a flawless fuel supply, venting of the fuel storage tank is absolutely essential. For, as fuel is used up, air has to be able to flow in behind it, since otherwise a vacuum would form in the tank, and the fuel flow would come to a stop. However, the tank also has to be vented so as to give the tank's contents sufficient opportunity to expand as it warms up. Also, when the tank is filled up, sufficient air has to be able to exit the tank so that the fuel being filled up does not bubble out of the filler pipe again.
Therefore, in such vehicles, increasingly tank venting systems are used in which the evaporating and excess fuel vapor is guided not into the open air but, via a venting line, into an active charcoal filter (AKF). This fuel vapor is stored temporarily in the AKF, and, during the operation of the motor vehicle, is guided via a clocked activatable electromagnetic tank venting valve (TEV) to the intake manifold of the internal combustion engine, and thus to combustion. This prevents emission of the environmentally harmful fuel vapors from the tank into the environment to the greatest extent, and at the same time the vapors supplied to the internal combustion engine are themselves used as fuel, whereby fuel usage is considerably reduced.
Based on the limited absorption volume of the active charcoal used in the AKF, one should intermittently regenerate the AKF. In order to do this, while the internal combustion engine is running, fresh air is drawn in via the AKF, and the fuel vapor removed in the process is supplied to the internal combustion engine as a mixture for combustion. The respective flushing quantity is controlled by the TEV via a performance characteristics adaptation using the parameters load and rotary speed, so that the running properties of the internal combustion engine are not impaired. A lambda control additionally monitors and regulates the regeneration. The lambda deviation resulting from this can then be drawn upon as a measurement of the loading state of the AKF.
In this connection, intensified legal regulations on the operation of internal combustion engines will apply in the future in some countries, such as the USA. Thereafter, it will be required for motor vehicles, in which volatile fuels like gasoline are used, that a possibly existing leakage in the entire fuel tank system be tracked down using an on-board arrangement.
Corresponding methods and devices for tank leakage diagnosis in a tank venting system of a motor vehicle are referred to, for example, in the U.S. Pat. No. 5,349,935, DE 196 36 431.0 A1, DE 198 09 384.5 A1 and DE 196 25 702 A1. In these, an overpressure is applied to the tank venting system, and a conclusion as to the presence of a leak is drawn from the subsequent course of the pressure. In the system of DE 196 36 431.0 A1, one may form a ram pressure between a pump and a reference leak, whereby the pump's rotary speed is lowered and the pump's current consumption increases. If the tank is leakproof, a higher pressure develops than when against the reference leak. The current consumption is consequently higher.
It may be observed that the tank leak diagnosis, instead of by the use of overpressure, may also be performed with the aid of underpressure.
A relatively high fuel degassing leads to erroneous measurements in tank leakage diagnosis. Therefore, as a measure of increased fuel degassing, a filtered loading factor of the AKF is used as a basis. The loading factor is calculated during travel, and filtered via a time constant. To do this, with the engine running, the TEV is controlled to open, and the deviation coming about in the lambda regulator, in this context, is recorded. Using the recorded deviation, together with the volume stream through the TEV that is also present in the engine control, the hydrocarbon (HC) concentration of the drawn in flushing volume stream is calculated. The HC concentration of the air drawn in through the AKF thus ascertained is valid as the measure of the magnitude of the AKF's loading. If the loading value exceeds a predefined threshold, the leakage diagnosis is interrupted or temporarily blocked.
Since the loading is a function not only of the magnitude of the fuel degassing, using the loading value alone no accurate statement can be made concerning the actual magnitude of the instantaneous degassing. Thus, even at a very great fuel degassing under certain travel conditions, the loading factor may be artificially kept low using a high purging rate. In such a case, the leakage diagnosis would be enabled because of the low loading factor and the low degassing supposed from this. In actual fact, however, because of the actually present high degassing, this would lead to erroneous results in the leakage diagnosis. In the case of the overpressure diagnosis method discussed above, the leakage quantities specified by law in the USA would not be met. In the underpressure methods also named, such an erroneous detection may lead to the mistaken diagnosis of a non-leakproof tank system.